1. Field of the Invention
The present invention relates to power semiconductor modules in which insulated substrates provided with power semiconductor elements made up of silicon (Si) or silicon carbide (SiC), such as IGBTs (insulated gate bipolar transistors) and MOS FETs, are mounted on a base plate, which is directly cooled by a cooling medium, and the present invention also relates to power semiconductor devices having such a module mounted therein.
2. Description of the Background Art
Typically, hybrid vehicles or electric vehicles use electric power conversion equipment (hereinafter, an “inverter device”) for driving a large capacity drive motor. A power semiconductor module, such as an IGBT module, is used as the inverter device to convert DC power to three-phase AC power to drive the drive motor, as well as to convert three-phase AC power to DC power to recycle energy. The power semiconductor module drives the drive motor while controlling high current, and therefore generates an extremely large amount of heat. On the other hand, the power semiconductor module to be mounted in hybrid vehicles or electric vehicles needs to be compact. Accordingly, in general, the power semiconductor module is cooled using a water-cooling structure with high cooling efficiency.
FIGS. 1A and 1B illustrate an example of the so-called indirect cooling structure that radiates heat by mounting a power semiconductor module on a mounting surface of a cooling jacket in which a cooling medium is circulated. The power semiconductor module 2 is bolted to the mounting surface 20 of the cooling jacket 20, as shown in FIG. 1A. Silicone grease for reducing thermal resistance is applied to the contact surface between the power semiconductor module 2 and the cooling jacket 20 to enhance thermal conductivity. The cooling medium is introduced from an intake port 21, and discharged from a discharge port 22 after passing through the inside of the cooling jacket 20. In addition, integrally-formed radiation fins 5 are arranged in the cooling jacket 20 at intervals of 1 to 2 mm to enhance radiation performance.
FIG. 1B is a cross-sectional view of a power semiconductor device 1 structured by combining the power semiconductor module 2 and the cooling jacket 20, as seen from direction B indicated in FIG. 1A. The power semiconductor module 2 is configured by soldering insulated substrates 9 having power semiconductor elements 8 mounted thereon to the top surface of a base plate 4, and furthermore, attaching a housing 24 to the base plate 4 so as to surround the power semiconductor elements 8. The insulated substrates 9 are each composed of: an insulated layer 10 made up of an insulated material, such as an insulated ceramic (e.g., aluminum nitride or silicon nitride), which has satisfactory thermal conductivity characteristics and a linear expansion coefficient close to that of silicon (Si); and metal layers 11a and 11b tightly provided on both surfaces of the insulated layer 10 and made up of copper, aluminum, or the like. The power semiconductor elements 8 are soldered on the upper metal layers 11a. In addition, the lower metal layer 11b is soldered on the top surface of the base plate 4.
The thickness of the metal layers 11a and 11b is determined considering the amount of current flowing through circuit patterns. In addition, the thickness of the base plate 4 is typically set at 3 to 4 mm to enhance its function as a thermal diffusion plate, thereby enhancing heat capacity. Furthermore, external terminals 25 are attached to the housing 24, and bonded to the power semiconductor elements 8 or the upper metal layers 11a via aluminum wiring. In the case where the housing 24 is conductive, the housing 24 is suitably insulated from the external terminals 25.
As described above, in the case of the indirect cooling-type power semiconductor device 1 shown in FIGS. 1A and 1B, silicone grease is applied to the contact surface between the power semiconductor module 2 and the cooling jacket 20 to reduce thermal resistance at the contact surface between them. However, the thermal conductivity of typically-used silicone grease is about 1 W/m·K, which is lower by two or more digits compared to the base plate 4 and the insulated substrates 9, and therefore heat generated by the power semiconductor elements 8 cannot be sufficiently conducted to the cooling jacket 20, resulting in poor radiation performance.
Therefore, for example, Japanese Laid-Open Patent Publication No. 2003-18178 has proposed a power semiconductor device 1 shown in FIG. 2. This power semiconductor device 1 includes a finned base plate 3, which is itself integrally formed with the radiation fins 5, so that the bottom surface of the finned base plate 3 is directly cooled by a cooling medium circulating in the cooling jacket 20.
Also, in the case of the power semiconductor device 1 shown in FIG. 2, the insulated substrates 9 and the finned base plate 3 are made up of materials each having a linear expansion coefficient close to that of the power semiconductor elements 8, thereby preventing any cracks in solder layers 12 located between the power semiconductor elements 8 and the insulated substrates 9, and between the insulated substrates 9 and the finned base plate 3.
For example, silicon (Si), which is a constituent material of the power semiconductor elements 8, has a linear expansion coefficient of about 3 ppm/° C., and therefore aluminum or silicon nitride substrates having a linear expansion coefficient of 3 to 5 ppm/° C. are used as the insulated substrates 9. Also, in consideration of ease of processing, an Al—SiC (aluminum-silicon carbide) composite plate having a linear expansion coefficient of 3 to 8 ppm/° C. is used as the finned base plate 3. The finned base plate 3 formed by the Al—SiC composite plate makes it possible to prevent any cracks in the solder layers 12, and furthermore, the radiation fins 5 exhibiting a complicated shape can be readily formed by metallic molding.